This invention relates to carriers for electrophotography, more particulaly, carriers with high electrical resistance used in a developer applied to an electrophotographic copier or a printer.
A method of copying by an electrophotographic copier or a printer consists of six processes. These processes include a uniformly charging process of a photosensitive member surface, an electrostatic latent image-forming process by irradiation of light corresponding to images of a manuscript, a developing process of electrostatic latent images on the photosensitive member by toners contained in a developer, a transferring process of toners on the photosensitive member to a copying thing, such as a paper, a fixing process of toners to the copying thing, and a cleaning process of the photosensitive member.
Known developers used in, for example, electrophotographic copiers include a two-component developer mainly containing insulating non-magnetic toners and magnetic carriers, and an one-component developer mainly containing insulating magnetic toners constituted of magnetic materials. The two-component developer is used in the following developing system. Charges are generated by friction between toners and carriers such as iron particles, magnetic brushes of the carriers are formed on a surface of a developing sleeve with built-in magnets to stir and transport the toners to a developing region, and then the charged toners are contacted with oppositely charged electrostatic latent images formed on a surface of a photosensitive member to develop the toners.
A developing system using a two-component developer brings about a defect such as white lines in developed images because of a hard ear of brush on account of strong suction force between each carrier particle on a surface of a developing sleeve. When the content of toners in a developer decreases, other troubles occur such as the disorder of electrosttic latent images, the deficit of developed images and the adherence of carriers to a part of electrostatic latent images on a photosensitive member and the like, because the charges of the electrostatic latent images on the photosensitive member tend to run away through the low specific resistant carriers of 10.sup.6 .OMEGA..cm or less, or charges are injected from the developing sleeve to the carriers.
In order to overcome the aforementioned problems relating to carriers consisting of magnetic materials such as iron particles, binder-type carriers have been proposed, in which magnetic particles with small particle size are blended with insulating binder resin.
Further, development at high speed, being desired recently, has brought a following new problem. Both a sleeve and magnets built in the sleeve are required to rotate at a high-speed in order to prevent irregular developments when binder-type carriers are used for the high-speed development. The high-speed rotation of magnets causes the increase of temperature of a sleeve on account of excessive eddycurrent.
Therefore, it has been proposed that binder-type carriers filled with a high content of magnetic particles are used in a copying machine assembled so that only the sleeve can rotate and the magnets built in the sleeve are fixed. Such carriers, however, have not enough magnetic properties to result in adherence of the carriers to a non-image part on the surface of an electrostatic latent image carrier. It is not preferred from the viewpoint of the retention of electrical resistance of carriers that the content of the magnetic particles in carriers becomes very high.